The present invention relates to an optical probe for the invasive measurement of blood parameters in a biologic circulatory system.
Probes for the invasive measurement of blood parameters usually consist of at least one sensor that is connected with an associated monitor via an optical fiber. Typically, such probes comprise between 1 and 3 sensors, e.g., intended for the measurement of blood gases such as partial oxygen pressure (pO.sub.2) or partial carbon dioxide pressure (pCO.sub.2), or for the measurement of the pH value of the blood. All these sensors have a similar mechanical construction. The optical fiber in each sensor ends with a gel containing a dye. The optical density or another optical parameter of the dye varies with the blood parameter to be measured. Light emitted by the associated monitor and transmitted via the optical fiber is fed into the gel and passes through it. The light is then fed back via the same or another optical fiber to the monitor, which contains a detector to measure light attenuation or changes in other optical parameters caused by the dye. This attenuation or change is a function of the blood parameter to be measured, and the relationship between attenuation, absorbance, or the change of another optical parameter and the blood parameter is well-known.
Usually, a reflector is positioned adjacent to the dye-containing gel, opposite to the optical fiber. In such a sensor, light transmitted through the optical fiber passes the gel, is reflected at the reflector, passes the gel again and is then transmitted back. In this environment, only one optical fiber is required for each sensor. Further, as the light passes the dye-containing gel twice, it is easier to detect any change in the optical characteristics of that dye. However, there are also other alternatives such as directing the light to a second optical fiber (when it has passed the gel) and feeding the second optical fiber back to the monitor. The key point in all of these cases is that the light has to pass the gel zone where its optical characteristics are altered.
The end of the fiber, the gel, and the reflector are surrounded by a semi-permeable or selective membrane (for example, a hydrogen ion permeable envelope in the case of a pH sensor). This membrane permits, on the one hand, only selected ions or molecules to reach the dye-containing gel; on the other hand, it has a mechanical function, namely to keep the gel in place.
In this description, and as it is usual in the art, the region of the dye-containing gel, together with the part of the membrane in this region, is called the "diffusion zone."
Optical probes as described herein usually comprise three or more sensors in order to measure various blood parameters with one probe. In these cases, the single optical fibers associated with the respective sensors are combined in a single cable for connection with the associated monitor. However, it is also possible to build an optical probe with one or two sensors only. Optical probes can be introduced into a patient's artery to measure--depending on the dye--various blood parameters such as pH, pO.sub.2 or pCO.sub.2, as described above. It is also possible to integrate further components such as a strain-relieving wire, an arterial pressure sensor, a temperature sensor, or the like into the probe. All of these components, including the sensors described above, will be referred to generically herein as sensing means.
Turning now in detail to the drawings, where like numerals designate similar elements, FIG. 1 depicts a typical system of the prior art for the invasive measurement of blood parameters, for example of the partial carbon dioxide pressure (pCO.sub.2) or the pH value. The light of an optical transmitter 1 is directed into an optical fiber 2 (see arrow 2a). Preferably, this optical fiber 2 is a glass fiber. Usually a train of light pulses is used, but this is not a strict requirement. The light passes an optical coupler 3 and reaches tip 4 of the sensor, the tip being intended for introduction into the artery of a patient. Tip 4 of the sensor contains a gel into which a dye such as phenol red is immobilized. The dye modifies at least one optical parameter, preferably the intensity, of the light in an amount dependant on the pCO.sub.2 (or, in other cases, the pO.sub.2 or the pH) value of the blood. The modified light is reflected into the same fiber 2 and, after passing through optical coupler 3, reaches an optical receiver 5 (see arrow 5a). It is understood that optical transmitter 1 and optical receiver 5 are incorporated in a monitor or other measuring instrument 8. Dashed line 6 indicates a releasable connection between the probe 7 and the monitor 8. Thus the optical probe consists of a multiplicity of sensors and the related number of optical fibers; preferably, it comprises 3 sensors responsive to pO.sub.2, pCO.sub.2 and pH, respectively.
The operation of a single sensor will now be explained by means of FIG. 2, which shows a longitudinal section through a pH sensor. The mechanical construction of the pH sensor is typical for sensors of this type; the pO.sub.2 and the pCO.sub.2 sensor would have a similar construction. As shown in FIG. 2, the pH sensor comprises a glass fiber 9 and an optical reflector 10. Optical reflector 10 is made of stainless steel. Between the optical fiber 9 and the reflector 10, a gel 11 is located. This gel is used to immobilize a dye such as phenol red, the optical characteristics of which vary with the blood parameter--in this case, pH--to be measured. The surface 10a of the optical reflector 10 facing the gel 11 is polished.
The sensor is surrounded by a semi-permeable or selective membrane 12 that is fastened on the sensor by means of a glue 13. As FIG. 2 depicts, the glue is only introduced at the distal end of the sensor (left side in FIG. 2) and at the very proximal end. The selective membrane 12 is permeable to the ions or gas molecules to be measured. In case of the pH sensor shown in FIG. 2, the selective membrane 12 is permeable to H.sup.+ ions.
FIG. 3 depicts a longitudinal section of the probe tip 14 of an optical probe comprising three sensors. A sheath 15 is closed at its outer end (proximal end) with a metal cap 16 and is connected, as shown by 17, with a tubing element 18. The connection between sheath 15 and tubing element 18 is secured by adhesive means. Tubing element 18 ends at a connector for connection to an appropriate monitor (not shown).
Sheath 15 contains three sensors, two of which are shown in FIG. 3, namely a pH sensor 19 and a pCO.sub.2 sensor 20. A third sensor, namely a pO.sub.2 sensor, is not shown in FIG. 3 as it is hidden behind pCO.sub.2 sensor 20.
Each of the sensors is connected with the associated monitor via an optical fiber, as shown by optical fiber 21 (which is surrounded by an appropriate envelope 22) for the case of pH sensor 19 and optical fiber 23 for the pCO.sub.2 sensor 20 (surrounded by envelope 24). The various sensors are fastened within sheath 15 by means of a silicone glue or adhesive 25.
Sheath 15 further comprises three openings, the first of which is labeled as 26 in FIG. 3, whereas the second opening 27 is hidden behind the pCO.sub.2 sensor 20. The third opening is not shown in FIG. 3; it is contained in the broken-away part. These openings ensure that, when the probe tip is introduced into a patient's artery, the sensors are in contact with the blood, thus allowing gas molecules and hydrogen ions to reach the sensors.
PCO.sub.2 sensor 20 further comprises a dye-containing gel 28 and an optical reflector 29. The region where dye-containing gel 28 is located is also called the "diffusion zone." Sensor 20 is, insofar as contained in sheath 15, surrounded by a semi-permeable membrane 30 that is fixed on optical fiber 23 and reflector 29 by means of a further glue or adhesive.
In similar manner, pH sensor 19 comprises a dye-containing gel 31, a reflector 32, and a semi-permeable membrane 33.
It is understood that the probe depicted in FIG. 3 is only one example of an invasive optical blood parameter probe. In other embodiments, the probe can comprise only one or two sensors, or even more elements, such as a strain relieving wire, all generically referred to as sensing means.
For a more detailed description of invasive fiber optic blood parameter measurement, reference is made to "Optical Fluorescence and its Application to an Intravascular Blood Gas Monitoring System," IEEE Transactions on Biomedical Engineering, Vol. BME-33, No. 2, February 1986, pages 117 et seq., and "A Miniature Fiber Optic pH Sensor for Physiological Use," Journal of Biomedical Engineering, May 1980, pages 141 et seq. Examples of the construction of an optical probe incorporating multiple sensors are described in European Patent Applications 279 004, 336 984, and 336 985, which are incorporated into the disclosure of this description by reference.
The latter European Patent Applications also describe a further advantageous component of the optical probe. This component is a sheath, usually a metal sheath, covering the proximal end of the sensors that keeps them together mechanically. The sheath is connected to a tubing element that covers the optical fibers (preferably, the tube is introduced into the sheath). Whether the sheath encloses a portion of the tube or the sheath and tube merely abut, they can be described as being in contact with each other.
Extensive tests have shown that the stability of the sensor readings in these types of optical probes is not always satisfactory. In particular, it has turned out that the sensor readings, once the sensors were used--either in a natural or an artificial blood vessel--differed from the readings of an unused sensor. By way of example, a probe was exposed to a certain chemical environment, but no mechanical stress, and its readings (pO.sub.2, pCO.sub.2 and pH) were recorded. Then, the same probe was exposed to mechanical stress, like the introduction into a natural or artificial blood vessel and subsequent withdrawal, and its readings were again recorded. In many cases, the second readings differed significantly from the first readings. This difference in the readings seemed to be a systemic error. Furthermore, the observed deviation did not disappear when the mechanical stress ended, i.e., the accuracy of the probe was permanently impaired.
This error affects the accuracy of the probe. Even worse, the error cannot be fully compensated for by recalibration of the sensor(s). Sensors of the type described herein are calibrated immediately before use. After introduction into a blood vessel, only a 1-point calibration (e.g., by comparison with the parameter value of the blood obtained by other methods such as in vitro blood sample analysis) can be done. Such a 1-point calibration can compensate for the constant offset, but it cannot compensate for a change in sensitivity caused by mechanical stress. In other words, neither a second point defining the slope of the sensor's characteristic curve, nor the slope itself, can be recalibrated.
Accordingly, there exists a need for an improved optical probe of the kind described above that avoids. minimizes, or reduces the reading errors occuring after the probe is subjected to mechanical stress.